If you mean the subdomain provided to you by x10Hosting, it doesn't expire. (Well, it sort of does, but only if x10Hosting itself fails to renew the main domain it's attached to. That's not your worry.) You are required to log into the single sign-on portal at https://x10hosting.com/sso/login/ at least once every 30 days, and you'll need to keep track of that yourself.